1. Field of the Invention
The present invention relates to a substrate processing method and a substrate processing apparatus, and more particularly to a substrate processing method for processing a substrate on which are formed a thermal oxide film and an oxide film containing impurities.
2. Description of the Related Art
There has been known a wafer (substrate) for semiconductor devices which has a thermal oxide film formed by a thermal oxidation process and an impurity-containing oxide film, such as a BPSG (Boron Phosphorous Silicate Glass) film, formed e.g. by a CVD process. The BPSG film is formed on a polysilicon film such that the polysilicon film is partially exposed, and functions as a hard mask during etching of the polysilicon film. On the other hand, the thermal oxide film forms a gate oxide film.
In this type of substrate, after etching the polysilicon film, it is required to selectively remove (etch) the BPSG film without removing (etching) the thermal oxide film.
Conventionally, an oxide film is etched using plasma generated from a CF-based gas. However, since the plasma generated from the CF-based gas etches not only the BPSG film, but also the thermal oxide film, it is difficult to secure a selection ratio of the BPSG film to the thermal oxide film, and hence the BPSG film cannot be selectively etched.
To overcome this problem, an etching method has been proposed which uses HF gas or a mixed gas of HF gas and H2O gas without turning the HF gas or the mixed gas into plasma (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. H06-181188). This method makes it possible to preferentially remove the impurity-containing oxide film by hydrofluoric acid produced by combining HF gas and H2O, so that the BPSG film can be selectively etched.
However, when HF gas or the mixed gas of HF gas and H2O gas is used to etch the BPSG film, etching residue is formed by reaction between SiO2 and hydrofluoric acid to be deposited on the surface of a semiconductor wafer. This residue causes a short circuit and the like trouble in a semiconductor device manufactured from the wafer.